There is increasing demand within the enterprise, the home and within cities to employ one wireless network to support both voice, video and data traffic. Currently, the “voice” network, e.g. the telephone system, is separate from the “data” network e.g. Internet connectivity and access to enterprise data over a Local Area Network (LAN). Convergence is, as the name implies, the ability to converge these two networks into one network, centrally managed by one access server servicing a network of relay and leaf nodes.
The challenge lies in providing—within the same wireless network—the ability to address potentially conflicting latency and throughput needs of diverse applications. For example, voice needs to be transmitted with low delay (latency). Occasionally lost voice packets, while undesirable, is not fatal for voice transmissions. Conversely, data transmissions mandate delivery of all packets and while low latency is desirable it is not essential. In essence transmission across the wireless network should ideally be driven by the needs of the application. The table below lists some types of applications and their latency requirements.
Delivery TypeDescriptionAsynchronousNo constraints on delivery time (“elastic”): e.g. emailSynchronousData is time-sensitive, but requirements flexible interms of delaysInteractiveDelays noticeable but not adversely affectingusability or functionalityIsochronousTime-sensitive: delays adversely affect usability.E.g. voice/videoMission criticalData delivery delays disables functionality
A wireless network provides service to a diverse set of applications, with varied latency requirements. One approach to make dumb wireless devices, that are nodes of the network, more application aware by implementing QoS (Quality of Service) reservation schemes dictated by the application server.
Changing the rate the queue is serviced can also be accomplished by specialized communications between wireless communication devices such as Access Point (AP) nodes and the access server to ensure that voice and data, for example, are serviced at different time intervals. Unfortunately, this adversely affects scalability and redundancy of the system: the access server is now micromanaging the network and has become a single point of failure. A paramount concern of any network is distributed control, especially a network handling voice, video and data.
Another shortcoming of a centralized approach—central control and central execution—is the cost of maintaining a central control point with all intelligence and control at one location and dumb communication devices distributed in the enterprise. The cost of the central control point is high, and the dumb access points are not any less expensive than smart access points—since the smarts is in the software. Thus a distributed approach is far less expensive—. In addition to being more cost effective a distributed approach is more fault tolerant and has built in fail-safe redundancy. The only way to get redundancy out of centralized approaches is to buy multiple central control points—an expensive approach.
Building a reliable wireless network comes with other constraints specific to wireless. Some routing paths may be best for voice and video, others for data. In Ethernet applications separate routing paths is easily accomplished. But in a wireless network, operating over radio, the cost of associating and disassociating with a relay node—to switch to new routing paths—is prohibitive. Multiple radios, supporting separate voice and data channels is possible but expensive. It is preferable, therefore, if each AP node can support both voice and data transmissions with a one “channel”.